Progress 12/15/00 to 12/31/04
Outputs
Siddharth G. Chatterjee (PI): (1) The predictions of a model of steady-state moisture transport in the thickness direction of a stack of paperboard sheets in a diffusion cup that accounted for the parallel diffusion of water vapor in the pore space and bound water in the fiber phase compared favorably with experimental data of the moisture transmission rate in a bleached kraft paperboard (BKP) stack, average moisture content of the stack, and relative humidity (RH) and sheet average moisture profiles in the stack. (2) A model of unsteady-state transport of moisture in the thickness direction of paper was formulated that considered the parallel diffusion of moisture in the pore and fiber phases. Using values of the effective water-vapor and bound-water diffusion coefficients of the BKP, estimated from steady-state moisture flux measurements, and no fitting parameter, the predictions of the dynamic model were in good agreement with measurements of the transient weight
change and RH profile of stacks of BKP sheets subjected to ramp changes of the external RH. (3) Some experimental data of the transient weight change of a BKP sheet under cyclic changes of the ambient RH were also taken. Bandaru V. Ramarao (co-PI):(1) Sorption transients under rapid humidity changes exhibited significant non-Fickian behavior even when the influence of external convection and rate of humidity change were eliminated as causes. Calculated sorption transients based on this concept showed good agreement with experimental observations for these rapid transients, which indicate that models for moisture diffusion should incorporate the ability to describe non-Fickian behavior. (2) In general, two types of models for moisture diffusion in paper can be recognized. Models of the first type treat paper as a homogeneous medium with moisture flux that is proportional to the moisture content gradient. Although useful in some instances this approach fails frequently because it
homogenizes the internal dynamics and relaxation processes occurring in paper. A review of recent studies, which have shown that a subtler approach which treats paper as a composite of fibers and void spaces is more successful at describing moisture transport dynamics, along with a generalization of this approach to three dimensions was performed. The parameters appearing in such models can be identified with the physical processes of diffusion through the void space and fiber matrix, supplemented by a local kinetic coefficient representing moisture interchange between the void and fiber phases. When the local moisture exchange coefficient takes on large values, the fibers and the void spaces are in local equilibrium with no net exchange of moisture, and the model reduces to the simpler Fickian diffusion model with non-linear moisture diffusivity equivalent to earlier models. (3) The effective diffusivity of handsheets made from bleached kraft softwood refined to different levels was
experimentally determined. The moisture transport parameters are the diffusivities in the void and fiber phases whose optimal values were estimated by a non-linear algorithm from experimental effective diffusivity data.
Impacts
Our investigation of steady and transient transport of moisture in paper indicates that the transport process can be conceptualized as one of parallel diffusion of water vapor in the pore space and bound water in the fiber phase of paper. Our work is one component of the knowledge base of moisture transport in cellulosic materials that should be useful in understanding the phenomena of accelerated creep of paperboard products under RH cycling; moisture transport in food, wood, and building materials; and drying of wood and paper.
Publications
- Gupta, H. and Chatterjee, S. G. 2003. Parallel diffusion of moisture in paper. Part 1: Steady-state conditions. Ind. Eng. Chem. Res. 42:6582-6592.
- Gupta, H. and Chatterjee, S. G. 2003. Parallel diffusion of moisture in paper. Part 2: Transient conditions. Ind. Eng. Chem. Res. 42:6593-6600.
- Gupta, H. 2003. Moisture transport in paper under steady and unsteady conditions investigated with a parallel diffusion concept. Ph.D. Thesis, SUNY College of Environmental Science and Forestry, Syracuse, NY 13210.
- Ramarao, B. V., Massoquete, A., Lavrykov, S. and Ramaswamy, S. 2003. Moisture diffusion inside paper materials in the hygroscopic range and characteristics of diffusivity parameters. Drying Tech. 21(10):2007-2056.
- Gupta, H. and Chatterjee, S. G. 2002. Facets of moisture diffusion in paper under steady-state conditions. Proc. 2002 Progress in Paper Physics Seminar, held at Skaneateles Falls, New York, September 8-13, p. 254-258, SUNY College of Environmental Science and Forestry, Faculty of Paper Science and Engineering, Empire State Paper Research Institute, Syracuse, NY 13210.
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